Pirate Skeleton is a Monster Set (Helm and Shoulder) available in light, medium or heavy armor. It drops in the Pirate Skeleton (unique) style.
The 1-piece set bonus adds 2975 Spell Resistance and 2975 Physical Resistance.
2-piece set bonus: When you take damage to your Health, you have a 8% chance to transform into a skeleton and gain Major Protection and Minor Defile for 12 seconds, reducing your damage taken by 30% but reducing your healing received and Health Recovery by 15%. This effect can occur once every 15 seconds.
The Shoulder can be acquired by using an Undaunted key on Glirion the Redbeard's chest. The Shoulder type, weight and trait are RNG based and can be from any dungeon this NPC gives Pledges for.
The Helm can be acquired from Blackheart Haven which is part of the base game and located in Bangkorai. The Helm is guaranteed to drop from the final boss in veteran mode only. The weight and trait are RNG based.
You can mix-and-match Monster Set shoulders and helms from different sets and add two 1-piece bonuses to your build, or wear a complete set to take advantage of the full 2-piece set bonus.